1) Treating someone differently, because of their gender identity or gender presentation.

2) Also, refusing to treat someone differently because of their gender identity or gender presentation.

3) Referring to a transgender person, or treating them, in any manner other than exactly what they would like you to.

1)
A: "Check out that guy in the dress; what a fruit."
B: "Dude, don't be so transphobic."

2)
C: "Hey, see that guy over there in the dress?"
B: "That's a girl."
C: "No, it's clearly a guy."
B: "She's wearing a dress, therefor you should refer to her as female."
C: "That doesn't make any sense."
B: "Dude, don't be so transphobic."